Event-related potentials (MMN and novelty P3) in permanent vegetative or minimally conscious states

Vegetative versus minimally conscious states: a study using TMS-EEG, sensory and event-related potentials

Differential diagnoses between vegetative and minimally conscious states (VS and MCS, respectively) are frequently incorrect. Hence, further research is necessary to improve the diagnostic accuracy at the bedside. The main neuropathological feature of VS is the diffuse damage of cortical and subcortical connections. Starting with this premise, we used electroencephalography (EEG) recordings to evaluate the cortical reactivity and effective connectivity during transcranial magnetic stimulation (TMS) in chronic VS or MCS patients. Moreover, the TMS-EEG data were compared with the results from standard somatosensory-evoked potentials (SEPs) and event-related potentials (ERPs). Thirteen patients with chronic consciousness disorders were examined at their bedsides. A group of healthy volunteers served as the control group. The amplitudes (reactivity) and scalp distributions (connectivity) of the cortical potentials evoked by TMS (TEPs) of the primary motor cortex were measured. Short-latency median nerve SEPs and auditory ERPs were also recorded. Reproducible TEPs were present in all control subjects in both the ipsilateral and the contralateral hemispheres relative to the site of the TMS. The amplitudes of the ipsilateral and contralateral TEPs were reduced in four of the five MCS patients, and the TEPs were bilaterally absent in one MCS patient. Among the VS patients, five did not manifest ipsilateral or contralateral TEPs, and three of the patients exhibited only ipsilateral TEPs with reduced amplitudes. The SEPs were altered in five VS and two MCS patients but did not correlate with the clinical diagnosis. The ERPs were impaired in all patients and did not correlate with the clinical diagnosis. These TEP results suggest that cortical reactivity and connectivity are severely impaired in all VS patients, whereas in most MCS patients, the TEPs are preserved but with abnormal features. Therefore, TEPs may add valuable information to the current clinical and neurophysiological assessment of chronic consciousness disorders.

Measuring affective reactivity in individuals with autism spectrum personality traits using the visual mismatch negativity event-related brain potential

The primary aim of this research was to determine how modulation of the visual mismatch negativity (vMMN) by emotionally laden faces is related to autism spectrum personality traits. Emotionally neutral faces served as the standard stimuli and happy and sad expressions served as vMMN-eliciting deviants. Consistent with prior research, it was anticipated that the amplitude of the vMMN would be increased for emotionally salient stimuli. Extending this finding, it was expected that this emotion-based amplitude sensitivity of the vMMN would be decreased in individuals with higher levels of autism spectrum personality traits as measured by the Adult Autism Spectrum Quotient (AQ). Higher AQ scores were associated with smaller amplitudes of the vMMN in response to happy, but not sad emotional deviants. The fact that higher AQ scores were associated with less sensitivity only to happy emotional expressions is interpreted to be consistent with the negative experience of social interactions reported by individuals who are high on the autism spectrum. This research suggests that the vMMN elicited by deviant emotional expressions may be a useful indicator of affective reactivity and may thus be related to social competency in Autism Spectrum Disorder (ASD).

Mindsight: diagnostics in disorders of consciousness

Diagnosis of patients with disorders of consciousness (comprising coma, vegetative state/unresponsive wakefulness syndrome, and minimally conscious state) has long been dependent on unstandardized behavioral tests. The arrival of standardized behavioral tools, and especially the Coma Recovery Scale revised, uncovered a high rate of misdiagnosis. Ancillary techniques, such as brain imaging and electrophysiological examinations, are ever more often being deployed to aid in the search for remaining consciousness. They are used to look for brain activity patterns similar to those found in healthy controls. The development of portable and cheaper devices will make these techniques more widely available.

A learning assessment procedure as a test supplement for monitoring progress with two post-coma persons with a diagnosis of vegetative state

OBJECTIVE

Evaluating a learning assessment procedure for monitoring progress with two post-coma adults with a diagnosis of vegetative state.

METHOD

ABABCBCB and ABABCB designs were used for the two participants, with A representing baseline, B intervention and C control conditions. Participants' activation of an optic microswitch by eyelid closure produced stimulation during B phases.

RESULTS

One participant increased responding during B phases and decreased it during the C condition, suggesting a non-reflective minimal level of consciousness. She showed P300 and mismatch negativity responses and scored at the vegetative level on the Coma Recovery Scale-Revised (CRS-R). The other participant increased responding during the initial B phases without decline during the first (viable) part of the C condition, suggesting a pre-conscious level. He showed indistinct P300 and mismatch negativity responses and vegetative-level scores on the CRS-R.

CONCLUSION

Learning data seemed reconcilable with neurophysiological measures and more positive than CRS-R scores.

Pain-related somatosensory evoked potentials and functional brain magnetic resonance in the evaluation of neurologic recovery after cardiac arrest: a case study of three patients

This case series investigates whether painful electrical stimulation increases the early prognostic value of both somatosensory-evoked potentials and functional magnetic resonance imaging in comatose patients after cardiac arrest. Three single cases with hypoxic-ischemic encephalopathy were considered. A neurophysiological evaluation with an electroencephalogram and somatosensory-evoked potentials during increased electrical stimulation in both median nerves was performed within five days of cardiac arrest. Each patient also underwent a functional magnetic resonance imaging evaluation with the same neurophysiological protocol one month after cardiac arrest. One patient, who completely recovered, showed a middle latency component at a high intensity of stimulation and the activation of all brain areas involved in cerebral pain processing. One patient in a minimally conscious state only showed the cortical somatosensory response and the activation of the primary somatosensory cortex. The last patient, who was in a vegetative state, did not show primary somatosensory evoked potentials; only the activation of subcortical brain areas occurred. These preliminary findings suggest that the pain-related somatosensory evoked potentials performed to increase the prognosis of comatose patients after cardiac arrest are associated with regional brain activity showed by functional magnetic resonance imaging during median nerves electrical stimulation. More importantly, this cases report also suggests that somatosensory evoked potentials and functional magnetic resonance imaging during painful electrical stimulation may be sensitive and complementary methods to predict the neurological outcome in the acute phase of coma. Thus, pain-related somatosensory-evoked potentials may be a reliable and a cost-effective tool for planning the early diagnostic evaluation of comatose patients.

Let live or let die after traumatic coma: Scrutinizing somatosensory evoked potentials

It is now firmly established that bilateral abolition of somatosensory evoked potentials (SEPs) after a nontraumatic coma has 100% specificity for nonawakening. In traumatic coma, a bilateral absence of the N20 components of SEPs does not implicate nonawareness. Comatose brain-injured patients should be systematically explored with auditory evoked potentials to check the functional integrity of another sensory pathway and the mesencephalic tegmento-tectal region on cerebral MRI should be carefully examined. Repeated evaluations during follow-up are also mandatory.

Arousal modulates auditory attention and awareness: insights from sleep, sedation, and disorders of consciousness

The interplay between attention and consciousness is frequently tested in altered states of consciousness, including transitions between stages of sleep and sedation, and in pathological disorders of consciousness (DoC; the vegetative and minimally conscious states; VS and MCS). One of the most widely used tasks to assess cognitive processing in this context is the auditory oddball paradigm, where an infrequent change in a sequence of sounds elicits, in awake subjects, a characteristic EEG event-related potential called the mismatch negativity, followed by the classic P300 wave. The latter is further separable into the slightly earlier, anterior P3a and the later, posterior P3b, thought to be linked to task-irrelevant "bottom-up" and task-oriented "top-down" attention, respectively. We discuss here the putative dissociations between attention and awareness in DoC, sedation and sleep, bearing in mind the recently emerging evidence from healthy volunteers and patients. These findings highlight the neurophysiological and cognitive parallels (and differences) across these three distinct variations in levels of consciousness, and inform the theoretical framework for interpreting the role of attention therein.

What is the specificity of the response to the own first-name when presented as a novel in a passive oddball paradigm? An ERP study

One's own first-name is a special stimulus: one's attention is more likely captured by hearing one's own first-name than by hearing another first-name. Previous event-related potential (ERP) studies demonstrated that this special stimulus produces differential responses both in active and in passive condition. Such results suggest that passively hearing one's own first-name triggers processing levels generally activated by the explicit detection of stimuli. This questions about the particular power of the own first-name to automatically orient attention, but no study investigated the specific response to this special stimulus in a paradigm designed to study automatic attention orienting. In this ERP study, we compared the responses elicited by the own first-name (OWN) and one unfamiliar first-name (OTHER) presented, rarely, randomly and at the same frequency among repetitive tones (i.e., as novel stimuli in an oddball paradigm) while subjects (N=36) were watching a silent movie with subtitles. We tested at what latency the responses to OWN and OTHER diverge, and whether OWN modulates the brain orienting response (novelty P3). Data analysis showed specific responses to OWN after 300 ms. OWN only evoked a central negativity (320 ms) and a parietal positivity (550 ms). However, OWN had no significant effect on the brain orienting response (260 ms). Our results confirm that the own first-name does elicit a late specific brain response. However, they challenge the idea that in passive condition, the own first-name is systematically more powerful than another first-name to orient attention when it is heard unexpectedly.

Usefulness of standard EEG in predicting the outcome of patients with disorders of consciousness after anoxic coma

Although standard EEG is performed routinely in patients with disorders of consciousness after coma, its prognostic value is still debated. The aim of the present study was to evaluate the role of standard EEG in predicting the recovery of cognitive functioning in patients affected by severe disturbances of consciousness after coma caused by cerebral anoxia. A standard EEG was recorded at admission to our Rehabilitation Department in 15 patients experiencing impaired consciousness because of cerebral anoxia. We quantified EEG abnormalities using the Synek scale (1988). Cognitive functioning was measured with the levels of cognitive functioning scale at the time of admission and after 3 months of recovery. EEG scores were significantly correlated with both levels of cognitive functioning scores at admission (P = 0.004) and change in levels of cognitive functioning score after 3 months (P < 0.001). The first correlation confirms the relationship between EEG and cognitive functioning, while the second correlation indicates the prognostic value of EEG in cognitive outcome. In conclusion, standard EEG is a simple and readily available tool with significant prognostic value in patients with disorders of consciousness after coma caused by cerebral anoxia.

Recovery of cortical effective connectivity and recovery of consciousness in vegetative patients

Patients surviving severe brain injury may regain consciousness without recovering their ability to understand, move and communicate. Recently, electrophysiological and neuroimaging approaches, employing simple sensory stimulations or verbal commands, have proven useful in detecting higher order processing and, in some cases, in establishing some degree of communication in brain-injured subjects with severe impairment of motor function. To complement these approaches, it would be useful to develop methods to detect recovery of consciousness in ways that do not depend on the integrity of sensory pathways or on the subject's ability to comprehend or carry out instructions. As suggested by theoretical and experimental work, a key requirement for consciousness is that multiple, specialized cortical areas can engage in rapid causal interactions (effective connectivity). Here, we employ transcranial magnetic stimulation together with high-density electroencephalography to evaluate effective connectivity at the bedside of severely brain injured, non-communicating subjects. In patients in a vegetative state, who were open-eyed, behaviourally awake but unresponsive, transcranial magnetic stimulation triggered a simple, local response indicating a breakdown of effective connectivity, similar to the one previously observed in unconscious sleeping or anaesthetized subjects. In contrast, in minimally conscious patients, who showed fluctuating signs of non-reflexive behaviour, transcranial magnetic stimulation invariably triggered complex activations that sequentially involved distant cortical areas ipsi- and contralateral to the site of stimulation, similar to activations we recorded in locked-in, conscious patients. Longitudinal measurements performed in patients who gradually recovered consciousness revealed that this clear-cut change in effective connectivity could occur at an early stage, before reliable communication was established with the subject and before the spontaneous electroencephalogram showed significant modifications. Measurements of effective connectivity by means of transcranial magnetic stimulation combined with electroencephalography can be performed at the bedside while by-passing subcortical afferent and efferent pathways, and without requiring active participation of subjects or language comprehension; hence, they offer an effective way to detect and track recovery of consciousness in brain-injured patients who are unable to exchange information with the external environment.

Measuring the fading consciousness in the human brain

PURPOSE OF REVIEW

Recent neuroimaging studies have provided novel insights on residual brain function in patients with disorders of consciousness, but also raised a number of ethical issues concerning the clinical management of these patients.

RECENT FINDINGS

Clinical studies have rated the Coma Recovery Scale as the most appropriate scale to accurately differentiate patients in a vegetative state from patients in a minimally conscious state. At the population level, a number of neuroimaging studies have provided evidence for more preserved brain activity patterns and cerebral tissue integrity in minimally conscious as compared to vegetative-state patients. However, the use of neuroimaging techniques to diagnose consciousness at the single-patient level remains challenging. In particular, it has been shown that whereas command-following functional MRI paradigms may sometimes detect residual awareness in patients that are behaviorally unresponsive, they can also produce negative results in patients that are communicative at the bedside.

SUMMARY

There is an urgent need of validation of functional MRI active paradigms on larger patient populations before they can be used in clinical routine. Further research on neural correlates of consciousness should hopefully allow using passive paradigms to assess the patients' conscious state without requiring their active collaboration.

Dogs cannot bark: event-related brain responses to true and false negated statements as indicators of higher-order conscious processing

The present study investigated event-related brain potentials elicited by true and false negated statements to evaluate if discrimination of the truth value of negated information relies on conscious processing and requires higher-order cognitive processing in healthy subjects across different levels of stimulus complexity. The stimulus material consisted of true and false negated sentences (sentence level) and prime-target expressions (word level). Stimuli were presented acoustically and no overt behavioral response of the participants was required. Event-related brain potentials to target words preceded by true and false negated expressions were analyzed both within group and at the single subject level. Across the different processing conditions (word pairs and sentences), target words elicited a frontal negativity and a late positivity in the time window from 600–1000 msec post target word onset. Amplitudes of both brain potentials varied as a function of the truth value of the negated expressions. Results were confirmed at the single-subject level. In sum, our results support recent suggestions according to which evaluation of the truth value of a negated expression is a time- and cognitively demanding process that cannot be solved automatically, and thus requires conscious processing. Our paradigm provides insight into higher-order processing related to language comprehension and reasoning in healthy subjects. Future studies are needed to evaluate if our paradigm also proves sensitive for the detection of consciousness in non-responsive patients.

Assessment of consciousness with electrophysiological and neurological imaging techniques

PURPOSE OF REVIEW

Brain MRI (diffusion tensor imaging and spectroscopy) and functional neuroimaging (PET, functional MRI, EEG and evoked potential studies) are changing our understanding of patients with disorders of consciousness encountered after coma such as the 'vegetative' or minimally conscious states.

RECENT FINDINGS

Increasing evidence from functional neuroimaging and electrophysiology demonstrates some residual cognitive processing in a subgroup of patients who clinically fail to show any response to commands, leading to the recent proposal of 'unresponsive wakefulness syndrome' as an alternative name for patients previously coined 'vegetative' or 'apallic'.

SUMMARY

Consciousness can be viewed as the emergent property of the collective behavior of widespread thalamocortical frontoparietal network connectivity. Data from physiological, pharmacological and pathological alterations of consciousness provide evidence in favor of this hypothesis. Increasing our understanding of the neural correlates of consciousness is helping clinicians to do a better job in terms of diagnosis, prognosis and finally treatment and drug development for these severely brain-damaged patients. The current challenge remains to continue translating this research from the bench to the bedside. Only well controlled large multicentric neuroimaging and electrophysiology studies will enable to identify which paraclinical diagnostic or prognostic test is necessary for our routine evidence-based assessment of individuals with disorders of consciousness.

Preserved feedforward but impaired top-down processes in the vegetative state

Frontoparietal cortex is involved in the explicit processing (awareness) of stimuli. Frontoparietal activation has also been found in studies of subliminal stimulus processing. We hypothesized that an impairment of top-down processes, involved in recurrent neuronal message-passing and the generation of long-latency electrophysiological responses, might provide a more reliable correlate of consciousness in severely brain-damaged patients, than frontoparietal responses. We measured effective connectivity during a mismatch negativity paradigm and found that the only significant difference between patients in a vegetative state and controls was an impairment of backward connectivity from frontal to temporal cortices. This result emphasizes the importance of top-down projections in recurrent processing that involve high-order associative cortices for conscious perception.

Inter-individual variability of oscillatory responses to subject's own name. A single-subject analysis

In previous studies event-related potentials and oscillations in response to subject's own name have been analyzed extensively on group-level in healthy subjects and in patients with a disorder of consciousness. Subject's own name as a deviant produces a P3. With equiprobable stimuli, non-phase-locked alpha oscillations are smaller in response to subject's own name compared to other names or subject's own name backwards. However, little is known about replicability on a single-subject level. Seventeen healthy subjects were assessed in an own-name paradigm with equiprobable stimuli of subject's own name, another name, and subject's own name backwards. Event-related potentials and non-phase locked oscillations were analyzed with single-subject, non-parametric statistics. No consistent results were found either for ERPs or for the non-phase locked changes of oscillatory activities. Only 4 subjects showed a robust effect as expected, that is, a lower activity in the alpha-beta range to subject's own name compared to other conditions. Four subjects elicited a higher activity for subject's own name. Thus, analyzing the EEG reactivity in the own-name paradigm with equiprobable stimuli on a single-subject level yields a high variance between subjects. In future research, single-subject statistics should be applied for examining the validity of physiologic measurements in other paradigms and for examining the pattern of reactivity in patients.

Preserved oscillatory response but lack of mismatch negativity in patients with disorders of consciousness

OBJECTIVE

The diagnostic validity of non-phase-locked oscillations (NPLOs) and mismatch negativity (MMN) in an oddball task for assessing attentional reactivity in patients with disordered consciousness was examined.

METHODS

Patients in a minimally conscious (MCS, n = 6) or vegetative (VS, n = 16) state and healthy controls (n = 15) were assessed. MMN and NPLOs were analyzed with single-subject, non-parametric statistics.

RESULTS

In 11 healthy controls and 2 VS patients, MMN was detected. More subjects showed NPLO differences in the alpha than in the theta or beta frequency ranges. In 14 healthy controls, 4 MCS patients, and 5 VS patients, lower amplitudes after deviants were found in the alpha frequency range. One healthy subject and one VS patient showed higher amplitudes after deviants.

CONCLUSIONS

Neither ERPs nor NPLOs could reliably distinguish MCS from VS patients. However, NPLOs were more sensitive than ERPs for detecting significantly different activity, and they possibly identified preserved processing better than ERPs.

SIGNIFICANCE

Intact neurophysiological attentional responses observed in the NPLOs of VS patients may indicate a need for other diagnostic techniques. Inter-individual differences in the direction of the effect should be considered as normal variance.

Refinement of metre perception--training increases hierarchical metre processing

Auditory metre perception refers to the ability to extract a temporally regular pulse and an underlying hierarchical structure of perceptual accents from a sequence of tones. Pulse perception is widely present in humans, and can be measured by the temporal expectancy for prospective tones, which listeners generate when presented with a metrical rhythm. We tested whether musical expertise leads to an increased perception and representation of the hierarchical structure of a metrical rhythm. Musicians and musical novices were tested in a mismatch negativity (MMN) paradigm for their sensitivity to perceptual accents on tones of the same pulse level (metre-congruent deviant) and on tones of a lower hierarchical level (metre-incongruent deviant). The difference between these two perceptual accents was more pronounced in the MMNs of the musicians than in those of the non-musicians. That is, musical expertise includes increased sensitivity to metre, specifically to its hierarchical structure. This enhanced higher-order temporal pattern perception makes musicians ideal models for investigating neural correlates of metre perception and, potentially, of related abstract pattern perception. Finally, our data show that small differences in sensitivity to higher-order patterns can be captured by means of an MMN paradigm.

EEG frequency analysis of responses to the own-name stimulus

OBJECTIVE

The neuronal response to hearing a subject's own (SON) compared with other names has been examined in healthy subjects as well as in patients with disorders of consciousness. So far, on electroencephalographic data, only event-related potentials (ERPs) were considered. In this study, we examined the frequency properties of SON.

METHODS

Data of 17 healthy subjects were processed for equiprobable stimuli of SON, other- and own-name backwards by calculating ERPs, evoked and induced activity for a period of 2000 ms from stimulus onset in the delta, theta, lower and upper alpha bands and averaging for four consequent temporal segments of 500 ms each.

RESULTS

For SON, the N1 component's amplitude was larger, while induced activity in the alpha band decreased in the second temporal segment (of 500-1000 ms). No differences between other- and own-name backwards were found.

CONCLUSIONS

The late reactivity may indicate responses to a stimulus after having recognised it. Alpha is known to play a role in attention and alertness. The results may reflect the fact that the SON stimulus enhances alertness.

SIGNIFICANCE

The findings correlate previous work about alertness and alpha activity with those about attention capturing of the SON stimulus. We suggest using frequency analysis in research on disorders of consciousness.